An optical scanning head of a scanning device is guided along a data track to read data stored on a rotating recording medium.
The scanning head directs a light beam onto the rotating recording medium through a lens attached to the scanning head to acquire the recorded information by scanning the data tracks applied to the recording medium. In order to avoid data reading errors, it is important in particular that the light beam is always guided exactly along the data tracks and is focused on the recording medium.
The housing of, e.g., a compact disc (CD) drive installed in a motor vehicle in particular is exposed to various shocks. If the playback device is subjected to vibrations, inertia forces act on the scanning device. At the same time, changes in the position of the laser beam relative to the data track occur since all components present tend to move with respect to each other due to the inertia forces. However, since the function of the device depends on a continuous exact positioning of the scanning head relative to the recording medium, there is in this case the danger of an interruption of function or data reading.
The tolerance for positional accuracy corresponds roughly to the geometric width of the data tracks. This is equivalent to approximately 1.6 μm in a compact disc and approximately 0.740 μm in a digital video disc (DVD).
The optical scanning head is moved on a radial track under the recording medium. Of critical significance for the vibration performance is the quality of the linear guidance of the optical scanning head on this radial track. In stationary operation, irregularities of guidance are compensated for by adjusting the optics. An electronic regulating device of the optical adjustment is adequately able to balance slowly changing deviations.
German Published Patent Application No. 196 42 343 describes a method used to regulate the focusing of a light beam of a scanning device directed onto a rotating recording medium to read data on the recording medium and the guidance of the light beam along the data tracks of the recording medium by at least one closed-loop control circuit. However, only translatory movements of the scanning device caused by vibrations in a lower frequency range up to approximately 100–200 Hz are correctable by this method. Vibrations in a frequency range above this limit cannot be compensated.
The manufacturing conditions cause fit tolerances among the individual components which cause them to undergo shocklike movements under vibration. However, the high-frequency components contained in the shocklike movements overtax the electronic regulating devices of the optics. This results in a malfunction of the read-write function.
These problems are illustrated in FIGS. 5a, 5b, and 6.
FIGS. 5a and 5b illustrate the presence of a fit tolerance 4 between a guide device 11 which guides scanning head 10 along a predefined track and a bearing device 12 for the support of guide device 11.
For cost reasons, scanning head 10 is guided on the other side only by a forked guide element.
For the sake of greater comprehensibility, pivot 20 and a section of data track 21 of recording medium 2 are shown in FIG. 5.
FIG. 6 shows a diagram of the vibration curves of the scanning head and of the entire recording device under acceleration events in the case of the presence of fit tolerances. A relative movement between the scanning head and the recording device due to inertia forces and the fit tolerance is clearly recognizable.